How to pack uv without breaking stacks of islands?
UV mapping is not my strong point, but when I get undesirable results I alternate between Smart UV unwrapping and the ordinary version.  When that fails I do a UV unwrap--> reset first, then a "Follow active quads" I think it's called.  That seems to divide the image up into even surface areas.  Once done I go back and redo it with smart or ordinary unwrapping.

This might sound weird to some (or many) but this procedure has actually got me around a few UV problems.